U.S. patent number 7,447,139 [Application Number 11/097,236] was granted by the patent office on 2008-11-04 for light-receiving element, optical head, optical recording/reproducing apparatus, and method of optical recording and reproduction.
This patent grant is currently assigned to TDK Corporation. Invention is credited to Koji Mishima, Teiichiro Oka, Giichi Shibuya, Kenji Yamaga, Daisuke Yoshitoku.
United States Patent |
7,447,139 |
Shibuya , et al. |
November 4, 2008 |
Light-receiving element, optical head, optical
recording/reproducing apparatus, and method of optical recording
and reproduction
Abstract
The invention relates to a light-receiving element for receiving
a reflection of laser light irradiated to a rotating multi-layer
recording medium having a plurality of information recording layers
stacked one over another and for converting the light into an
electrical signal, an optical head having the element for recording
information in the multi-layer recording medium or reproducing
information recorded therein, and an optical recording/reproducing
apparatus and a method of optical recording and reproduction. The
invention provides a light-receiving element, an optical head, an
optical recording/reproducing apparatus, and a method of optical
recording and reproduction which make it possible to eliminate a
noise component superimposed on reflected light from a multi-layer
recording medium to reproduce an RF signal of high quality. The
light receiving element receives a reflection of laser light
irradiated through an objective lens to a rotating multi-layer
recording medium having a plurality of information recording layers
stacked one over another through a return path optical system and
converts the reflected light into an electrical signal. The element
has a first light-receiving section having a circular
light-receiving region and a second light-receiving section
disposed adjacent to the outer circumference of the first
light-receiving section in the form of a circle concentric
therewith.
Inventors: |
Shibuya; Giichi (Tokyo,
JP), Oka; Teiichiro (Tokyo, JP), Mishima;
Koji (Tokyo, JP), Yoshitoku; Daisuke (Tokyo,
JP), Yamaga; Kenji (Tokyo, JP) |
Assignee: |
TDK Corporation (Tokyo,
JP)
|
Family
ID: |
35136278 |
Appl.
No.: |
11/097,236 |
Filed: |
April 4, 2005 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20050237904 A1 |
Oct 27, 2005 |
|
Foreign Application Priority Data
|
|
|
|
|
Apr 15, 2004 [JP] |
|
|
2004-120671 |
|
Current U.S.
Class: |
369/120;
369/44.42; G9B/7.135; G9B/7.168 |
Current CPC
Class: |
G11B
7/133 (20130101); G11B 7/24038 (20130101) |
Current International
Class: |
G11B
7/00 (20060101) |
Field of
Search: |
;369/94,44.41,44.42,120 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
02-158930 |
|
Jun 1990 |
|
JP |
|
A-05-036146 |
|
Feb 1993 |
|
JP |
|
09-050640 |
|
Feb 1997 |
|
JP |
|
B2 2624255 |
|
Apr 1997 |
|
JP |
|
10-261238 |
|
Sep 1998 |
|
JP |
|
A 11-016200 |
|
Jan 1999 |
|
JP |
|
2002-131618 |
|
May 2002 |
|
JP |
|
A 2002-319177 |
|
Oct 2002 |
|
JP |
|
2005-122795 |
|
May 2005 |
|
JP |
|
Primary Examiner: Tran; Thang V
Attorney, Agent or Firm: Oliff & Berridge, PLC
Claims
What is claimed is:
1. A light-receiving element for receiving a reflection of laser
light irradiated through an objective lens to a rotating
multi-layer recording medium having a plurality of information
recording layers stacked one over another through a return path
optical system and converting the reflected light into an
electrical signal, the light-receiving element comprising: a first
light-receiving section having a circular light-receiving region;
and a second light-receiving section disposed adjacent to an outer
circumference of the first light-receiving section, wherein a
following relational expression is satisfied:
.pi.(0.5.lamda./(NA/.beta.)).sup.2.ltoreq.S.sub.1.ltoreq..pi.(0.24d.beta.-
NA/n).sup.2 where .lamda. represents the wavelength of the laser
light; d represents the distance between recording layers of the
multi-layer recording medium; n represents the refractive index of
a light-transmitting layer between the recording layers; NA
represents the numerical aperture of the objective lens; .beta.
represents the lateral magnification of the return path optical
system; and S.sub.1 represents the surface area of the
light-receiving region of the first light-receiving section.
2. A light-receiving element according to claim 1, wherein a
frequency band of an electrical signal output from the first
light-receiving section includes a frequency band of an electrical
signal output from the second light-receiving section.
3. A light-receiving element according to claim 1, wherein a
light-receiving region of the second light-receiving section is
formed in a form of a concentric circle around the outer
circumference of the first light-receiving section.
4. A light-receiving element according to claim 3, wherein a
following relational expression is satisfied:
S.sub.m.gtoreq..pi.(1.1(m-1)d.beta.NA/n).sup.2 where S.sub.m
represents a sum of the surface area of the light-receiving region
of the first light-receiving section and the surface area of the
light-receiving region of the second light-receiving section up to
the light-receiving region that is in an m-th place (m.gtoreq.2)
when counted from the light-receiving region of the first
light-receiving section.
5. A light-receiving element according to claim 1, further
comprising a differential amplification circuit having a
non-inverting input terminal to which the electrical signal output
from the first light-receiving section is input and an inverting
input terminal to which a noise signal output from the second
light-receiving section is input, the differential amplification
circuit performing a differential operation between the electrical
signal and the noise signal.
6. A light-receiving element according to claim 5, wherein the
noise signal originates in an inter-layer crosstalk that occurs
between reflected light from a recording layer of the multi-layer
recording medium to be reproduced and reflected light from a
recording layer of the multi-layer recording medium other than the
recording layer to be reproduced.
7. A light-receiving element according to claim 5, wherein the
electrical signal includes an RF signal including information
recorded on the recording layer to be reproduced and the noise
signal.
8. A light-receiving element according to claim 5, wherein the
differential amplification circuit has a noise signal selection
circuit for selecting a noise signal output from the
light-receiving region of the second light-receiving section in the
m-th (m.gtoreq.2) place when counted from the first light-receiving
section toward the outer circumference of the element.
9. A light-receiving element according to claim 8, wherein the
differential amplification circuit has a switch for switching the
electrical signal to input the signal to either the non-inverting
input terminal or the inverting input terminal, an arithmetic
circuit section for performing a calculation on the electrical
signal and the noise signal selected by the noise signal selection
circuit, and an output terminal from which a signal obtained by the
calculation at the arithmetic circuit section is output.
10. A light-receiving element according to claim 9, wherein the
differential amplification circuit has an input terminal to which
an output signal from a logic circuit is input and the opening and
closing of the switch is controlled by a logical input from the
logic circuit.
11. An optical head comprising a light-receiving element according
to claim 1.
12. An optical head according to claim 11, wherein the
light-receiving element is used as a light-receiving element for
monitoring laser power.
13. An optical recording/reproducing apparatus comprising an
optical head according to claim 11.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light-receiving element for
receiving a reflection of laser light irradiated to a rotating
multi-layer recording medium having a plurality of information
recording layers stacked one over another and converting the
reflected light into an electrical signal, an optical head having
the element and recording information in a multi-layer recording
medium or reproducing information recorded therein, an optical
recording/reproducing apparatus, and a method of optical recording
and reproduction.
2. Description of the Related Art
An optical recording/reproducing apparatus has an optical head
which is formed along the circumferential direction of, for
example, a disk-shaped optical recording medium (optical disk) and
which records information in predetermined regions of a plurality
of tracks formed in the radial direction of the optical recording
medium or reproduces information recorded in predetermined regions
of the tracks. Optical heads include recording-only types which are
used only for recording information in an optical recording medium,
reproduction-only types which are used only for reproducing
information, and recording/reproduction types which can be used for
both recording and reproduction. Therefore, apparatus loaded with
those types of heads respectively constitute optical recording
apparatus, optical reproducing apparatus, and optical
recording/reproducing apparatus. In this specification, the term
"optical recording/reproducing apparatus" will be used as a general
term that implies all of those apparatus.
Patent Documents 1 and 2 disclose optical heads and optical disk
apparatus for performing reproduction from a multi-layer optical
disk having a plurality of recording layers for recording
information. In the optical heads and optical disk apparatus, light
reflected from a multi-layer optical disk (return light) is split
into two or more optical paths which are converged on separate
light-receiving sections. The light-receiving section for a first
beam of light receives the entire light beam, and the
light-receiving section for a second beam of light receives a
central part or peripheral part of the light beam. Electrical
signals obtained by photoelectrical conversion at the first and
second light-receiving sections, respectively, are subjected to a
differential operation at a differential amplification circuit to
reproduce a reproduction signal (RF signal). A method is also
disclosed, in which the second light beam is used as a focus error
signal. The method for reducing inter-layer crosstalks in a
multi-layer optical disk using a differential amplification circuit
is advantageous in that it serves the purpose of reducing the size
of an optical head and an optical disk apparatus because the method
involves no focal point in a return path optical system except a
light-receiving section and therefore allows an optical system to
have a short optical path unlike the method disclosed in Patent
Document 3 for reducing inter-layer crosstalks by using a confocal
optical head.
[Patent Document 1] JP-A-1999-16200
[Patent Document 2] JP-A-2002-319177
[Patent Document 3] Japanese Patent No. 2624255
According to the method disclosed in Patent Document 1, however,
one of two light beams split to perform a differential operation
must be received by a light-receiving element disposed in a
position where a reflected light beam from a recording layer to be
reproduced (reproduced layer) converges, and the other light beam
must be received by a light-receiving element disposed in a
position where an inner circumferential part of a reflected light
beam from a recording layer adjacent to the reproduced layer
converges. Since it is not possible to dispose both of the elements
in one optical path, a problem arises in that optical elements are
required redundantly. According to the method disclosed in Patent
Document 2, a light beam is split by two light-receiving elements
into inner and outer circumferential parts, and a resultant
differential signal is output as an RF signal. The method serves
the purpose of providing a compact optical head because a light
beam can be split by two light-receiving elements located close to
each other. However, no sufficient study has been made on the size
of the light-receiving elements and the configuration of the
light-receiving elements to perform the splitting. There is a
limitation on electrical signals to be subjected to a differential
operation in that they must originate only in reflected beams from
particular layers (adjoining layers), and no study has been made on
how to reduce inter-layer crosstalks in a multi-layer optical disk
having three or more recording layers effectively.
In a multi-layer optical disk having three or more layers,
reflected light (return light) from a layer adjacent to a layer
under reproduction does not necessarily constitute the inter-layer
crosstalk having the most significant influence on the reproduction
of an RF signal. FIG. 9 is an illustration for explaining
inter-layer crosstalks which can generate noise signals in a
multi-layer optical disk. Referring to FIG. 9, let us assume that a
recording layer L4 is a reproduced layer from which recorded
information is to be reproduced; recording layers L0 and L3 are
recorded regions; and a recording layer L2 is an unrecorded region.
In general, the quantity of return light attributable to reflection
at an unrecorded region is greater than the quantity of return
light from a recorded region.
When light irradiating the multi-layer optical disk is focused on
the recording layer L4 to reproduce the information recorded on the
recording layer L4, reflected light from the recording layer L3 is
focused near the recording layer L2. Since the recording layer L3
and the recording layer L2 are a recorded region and an unrecorded
region, respectively, the resultant inter-layer crosstalk has a
relatively small influence on the signal reproduced from the
recording layer L4. Reflected light from the recording layer L2 is
focused near the recording layer L0, and the recording layer L0 is
a recorded region. Therefore, the resultant inter-layer crosstalk
has a more significant influence on the signal reproduced from the
recording layer L4. Therefore, the reflected light from the
recording layer L2 that is further from the recording layer L4
constitutes a more significant inter-layer crosstalk than the
reflected light from the recording layer L3 adjacent to the
recording layer L4, the inter-layer crosstalk having the most
significant influence on an RF signal from the recording layer L4
to be reproduced and constituting a source of noises. Thus, the
recording layer that generates the reflected light by which
inter-layer crosstalk occurs acting as the most dominant noise
source varies depending on whether each of the recording layers in
the optical path is recorded or not. In this case, the return light
includes noise components from other recording layers to be
eliminated as reflected light components which are defocused from
the position in focus by an amount that is twice an optical
distance between the layers. No consideration on this point is seen
in Patent Document 2.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a light-receiving
element, an optical head, an optical recording/reproducing
apparatus, and a method of optical recording and reproduction which
make it possible to eliminate noise components superimposed on
reflected light from a multi-layer recording medium and to thereby
reproduce an RF signal of high quality.
The above-described object is achieved by a light receiving element
characterized in that it receives a reflection of laser light
irradiated through an objective lens to a rotating multi-layer
recording medium having a plurality of information recording layers
stacked one over another through a return path optical system and
converts the reflected light into an electrical signal, the element
comprising a first light-receiving section having a circular
light-receiving region and a second light-receiving section
disposed adjacent to an outer circumference of the first
light-receiving section.
The invention provides a light-receiving element characterized in
that a frequency of an electrical signal output from the first
light-receiving section includes a frequency of an electrical
signal output from the second light-receiving section.
The invention provides a light-receiving element characterized in
that it satisfies a following relational expression:
.pi.(0.5.lamda./(NA/.beta.)).sup.2.ltoreq.S.sub.1.ltoreq..pi.(0.24d.beta.-
NA/n).sup.2 where .lamda. represents the wavelength of the laser
light; d represents the distance between recording layers of the
multi-layer recording medium; n represents the refractive index of
a light-transmitting layer between the recording layers; NA
represents the numerical aperture of the objective lens; .beta.
represents the lateral magnification of the return path optical
system; and S.sub.1 represents the surface area of the
light-receiving region of the first light-receiving section.
The invention provides a light-receiving element characterized in
that a light-receiving region of the second light-receiving section
is formed in a form of a concentric circle around the outer
circumference of the first light-receiving section.
The invention provides a light-receiving element characterized in
that it satisfies a following relational expression:
S.sub.m.gtoreq..pi.(1.1(m-1)d.beta.NA/n).sup.2 where S.sub.m
represents a sum of the surface area of the light-receiving region
of the first light-receiving section and the surface area of the
light-receiving region of the second light-receiving section up to
the light-receiving region that is in an m-th place (m.gtoreq.2)
when counted from the light-receiving region of the first
light-receiving section.
The invention provides a light-receiving element characterized in
that it comprises a differential amplification circuit having a
non-inverting input terminal to which the electrical signal output
from the first light-receiving section is input and an inverting
input terminal to which a noise signal output from the second
light-receiving section is input, the differential amplification
circuit performing a differential operation between the electrical
signal and the noise signal.
The invention provides a light-receiving element characterized in
that the noise signal originates in an inter-layer crosstalk that
occurs between reflected light from a recording layer of the
multi-layer recording medium to be reproduced and reflected light
from a recording layer of the multi-layer recording medium other
than the recording layer to be reproduced.
The invention provides a light-receiving element characterized in
that the electrical signal includes an RF signal including
information recorded on the recording layer to be reproduced and
the noise signal.
The invention provides a light-receiving element characterized in
that the differential amplification circuit has a noise signal
selection circuit for selecting a noise signal output from the
light-receiving region of the second light-receiving section in the
m-th (m.gtoreq.2) place when counted from the first light-receiving
section toward the outer circumference of the element.
The invention provides a light-receiving element characterized in
that the differential amplification circuit has a switch for
switching the electrical signal to input the signal to either the
non-inverting input terminal or the inverting input terminal, an
arithmetic circuit section for performing a calculation on the
electrical signal and the noise signal selected by the noise signal
selection circuit, and an output terminal from which a signal
obtained by the calculation at the arithmetic circuit section is
output.
The invention provides a light-receiving element characterized in
that the differential amplification circuit has an input terminal
to which an output signal from a logic circuit is input and in that
the opening and closing of the switch is controlled by a logical
input from the logic circuit.
The above-described object is achieved by an optical head
characterized in that it comprises a light-receiving element
according to the invention.
The invention provides an optical head characterized in that the
light-receiving element is used as a light-receiving element for
monitoring laser power.
The above-described object is achieved by an optical
recording/reproducing apparatus characterized in that it comprises
an optical head according to the invention.
The above-described object is achieved by a method of optical
recording and reproduction characterized in that it comprises the
steps of irradiating a rotating multi-layer recording medium having
a plurality of information recording layers stacked one over
another with laser light, receiving reflected light of the laser
light reflected by a recording layer of the multi-layer recording
medium to be reproduced and converting the reflected light into an
electrical signal at a first light-receiving section having a
circular light-receiving region, receiving reflected light of the
laser light reflected by a recording layer other than the recording
layer to be reproduced and converting the reflected light into a
noise signal at a second light-receiving section disposed adjacent
to the outer circumference of the first light-receiving section,
and extracting an RF signal by performing a differential operation
between the electrical signal and the noise signal.
The invention provides a method of optical recording and
reproduction characterized in that the noise signal originates in
an inter-layer crosstalk that occurs between reflected light from
the recording layer to be reproduced and the reflected light from
the recording layer other than the recording layer to be
reproduced.
The invention provides a method of optical recording and
reproduction characterized in that the electrical signal includes
an RF signal including information recorded on the recording layer
to be reproduced and the noise signal.
The invention provides a method of optical recording and
reproduction characterized in that the noise signal is extracted
from each of a plurality of recording layers other than the
recording layer to be reproduced and in that a differential
operation is performed between any selected one of the plurality of
extracted noise signals and the electrical signal to extract the RF
signal.
The invention makes it possible to provide a light-receiving
element, an optical head, and an optical recording/reproducing
apparatus which allow a noise component superimposed on reflected
light from a multi-layer recording medium to be eliminated to
reproduce an RF signal of high quality.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic configuration of an optical head 1
according to an embodiment of the invention;
FIG. 2 shows a schematic configuration of a light-receiving section
of a light-receiving element 25 according to the embodiment of the
invention;
FIG. 3 is a graph showing the intensity distribution and optical
energy distribution of focused light received on a light-receiving
surface of a first light-receiving section 27 through a return path
optical system of the optical head 1 according to the embodiment of
the invention;
FIG. 4 is a graph showing the intensity distribution and optical
energy distribution of defocused light received on light-receiving
surfaces of the first light-receiving section 27 and a second
light-receiving section 29 through the return path optical system
of the optical head 1 according to the embodiment of the
invention;
FIG. 5 shows a differential amplification circuit 31 for extracting
an RF signal including information recorded on a multi-layer
optical disk 15 from an electrical signal output by the
light-receiving element 25 according to the embodiment of the
invention;
FIG. 6 shows a schematic configuration of an optical
recording/reproducing apparatus 50 according to the embodiment of
the invention;
FIG. 7 shows a schematic configuration of light-receiving sections
which are a modification of the light-receiving element 25
according to the embodiment of the invention;
FIG. 8 shows a differential amplification circuit 31 which is a
modification of the optical head 1 according to the embodiment of
the invention; and
FIG. 9 is an illustration for explaining inter-layer crosstalks
which can generate noise signals in a multi-layer optical disk
according to the related art.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A description will now be made with reference to FIGS. 1 to 6 on a
light-receiving element, an optical head, an optical
recording/reproducing apparatus, and a method of optical recording
and reproduction according to an embodiment of the invention.
First, a schematic configuration of an optical head according to
the present embodiment will be described with reference to FIGS. 1
and 2. An optical head 1 has a laser diode 3 to serve as a
laser-emitting element which emits laser light. The laser diode 3
can emit laser light having a different optical intensity for each
of recording and reproduction based on a control voltage from a
controller (not shown).
A polarization beam splitter 5 is disposed in a predetermined
position on a light-emitting side of the laser diode 3. A
quarter-wave plate 7, a collimator lens 9, and an objective lens 13
are disposed in the order listed on a light-transmitting side of
the polarization beam splitter 5 as viewed from the laser diode 3.
The collimator lens 9 is provided to transform a divergent pencil
of rays from the laser diode 3 into a parallel pencil of rays and
to guide the parallel pencil of rays to the objective lens 13, and
to transform a parallel pencil of rays from the objective lens 13
into a convergent pencil of rays and to guide a convergent pencil
of rays to a light-receiving element 25. The objective lens 13 is
provided to converge the parallel pencil of rays from the
collimator lens 9 on a predetermined recording layer of a
multi-layer optical disk (multi-layer recording medium) 15 having a
plurality of recording layers to form a reading spot on the same
and to transform reflected light from the multi-layer optical disk
15 into a parallel pencil of rays and to guide the parallel pencil
of rays to the collimator lens 9.
A sensor lens 17, a cylindrical lens 21, and a light-receiving
element 25 are disposed in the order listed on a light-reflecting
side of the polarization beam splitter 5 as viewed from the
quarter-wave plate 7. A power-monitoring photodiode 11 for
measuring the optical intensity of laser light emitted by the laser
diode 3 is disposed on a light-reflecting side of the polarization
beam splitter 5 as viewed from the laser diode 3.
The sensor lens 17 serves as a reflected light focus position
adjusting section for optically adjusting the position in focus of
an optical beam reflected by the multi-layer optical disk 15. The
sensor lens 17 enlarges reflected light at a predetermined optical
magnification and focuses the same on the light-receiving element
25 through the cylindrical lens 21. An electrical signal obtained
by photoelectric conversion at the light-receiving element 25 is
input to a differential amplification circuit 31 (see FIG. 5), and
an RF signal is reproduced from the electrical signal at the
differential amplification circuit 31.
FIG. 2 shows a configuration of light-receiving sections of the
light-receiving element 25. As shown in FIG. 2, the light-receiving
element 25 includes a first light-receiving section 27 having a
circular light-receiving region and a second light-receiving
section 29 which is formed around the outer circumference of the
first light-receiving section 27 in the form of a concentric circle
that is concentric with the circular light-receiving region of the
first light-receiving section 27.
The first light-receiving section 27 is disposed such that the
center of the circular light-receiving region thereof substantially
coincides with the optical axis of a return path optical system
which passes through the objective lens 13, collimator lens 9, the
quarter-wave plate 7, the beam splitter 5, the sensor lens 17, and
the cylindrical lens 21 in the order listed. The first
light-receiving section 27 is also disposed such that the
light-receiving surface is located in the focusing position of
reflected light (reproduced signal light) from a predetermined
recording layer from which information is to be reproduced
(hereinafter referred to as "reproduced layer") among the plurality
of recording layers of the multi-layer optical disk 15. The surface
area (represented by S.sub.1) of the light-receiving region is
optimized to allow the reproduced signal light to be received with
a necessary and sufficient intensity. The first light-receiving
section 27 also receives reflected light other than the signal
light reflected from other recording layers (noise signal light),
although in a very small quantity. The second light-receiving
section 29 receives noise signal light reflected from recording
layers other than the reproduced layer.
An insulation region 26 is provided between the light-receiving
regions of the first light-receiving section 27 and the second
light-receiving section 29 to keep them insulated from each other.
A wiring area 28 is formed in the light-receiving region of the
second light-receiving section 29 so as to extend in the radial
direction from the center of the region. The wiring area 28 is
provided to form a wiring for connecting the first light-receiving
section 27 and the differential amplification circuit 31 (see FIG.
5).
Operations of the optical head 1 will now be described with
reference to FIGS. 1 and 2. Divergent laser light emitted by the
laser diode 3 impinges upon the polarization beam splitter 5. A
Linearly polarized component of the light in a predetermined
polarization direction is transmitted by the polarization beam
splitter 5 to impinge upon the quarter-wave plate 7. A linearly
polarized component orthogonal to the above-mentioned polarization
direction is reflected to impinge upon the power monitoring
photodiode 11, and the intensity of the laser light is thus
measured.
The linearly polarized light which has entered the quarter-wave
plate 7 is transformed into circularly polarized light by being
transmitted through the quarter-wave plate 7. The circularly
polarized light is collimated by the collimator lens 9, transmitted
by the collimator lens 9, and converged by the objective lens 13.
The light is collected and reflected on a layer from which
information is to be reproduced among the plurality of recording
layers of the multi-layer optical disk 15. At this time, the light
is also reflected by recording layers other than the reproduced
layer. The circularly polarized light reflected by a plurality of
recording layers of the multi-layer optical disk 15 is collimated
by the objective lens 13 and transmitted by the collimator lens 9
to impinge upon the quarter-wave plate 7. By being transmitted
through the quarter-wave plate 7, the circularly polarized light is
transformed into linearly polarized light which impinges upon the
polarization beam splitter 5, the polarization direction of the
linearly polarized light being at a 90.degree. rotation from that
of the initial linearly polarized light. The linearly polarized
light is reflected by the polarization beam splitter 5 to impinge
upon the sensor lens 17.
After being transmitted by the sensor lens 17, the light is
converged on the light-receiving element 25 through the cylindrical
lens 21. The first light-receiving section 27 of the
light-receiving element 25 receives not only the signal light from
the reproduced layer but also the reflected light from the other
recording layers which can cause inter-layer crosstalks, although
the latter is in a very small quantity. The second light-receiving
section 29 receives the reflected light including noise signals
from the recording layers other than the reproduced layer.
A description will now be made with reference to FIGS. 3 and 4 on
optimization of the shapes and surface areas of the first and
second light-receiving sections 27 and 29 of the light-receiving
element 25. FIG. 3 shows the intensity distribution and optical
energy distribution of focused light that is received on the
light-receiving surface of the first light-receiving section 27
through the return path optical system of the optical head 1. In
FIG. 3, distances (.mu.m) from the center of the light-receiving
region of the first light-receiving section 27 in the radial (r)
direction thereof are plotted along the abscissa axis, and the
intensity I(r) of the focused light in each position in the radial
(r) direction and an integrated value of optical intensities
(optical energy P(r)) from the center of the region at each of the
radial positions are plotted along the ordinate axis in a
normalized manner. In FIG. 3, the intensity I(r) of the focused
light is represented by the curve connecting the solid diamonds,
and the optical energy P(r) is represented by the curve connecting
the solid squares. In the present embodiment, it is assumed that
the laser has a wavelength .lamda. of 405 nm and that a
light-receiving section on the side of the return path (which
depends on the power of the collimator lens 9 and the sensor lens
17) has a numerical aperture NA' that is 0.1.
It is known that the intensity distribution of a light spot in a
position in focus is obtained by performing Fourier integration of
the intensity distribution of the light incident upon the lens
throughout the aperture of the lens and that the extent of the
distribution (the diameter of the spot) is proportionate to the
ratio of the laser wavelength .lamda. to the numerical aperture NA'
of the light-receiving section on the side of the return path
(reference document: "General Compilation of Optical Memory and
Photomagnetic Memory Technologies", supervised by Yoshifumi Sakurai
and Shizuo Tatsuoka, Science Forum pp. 91-, 1983). That is, when
the laser wavelength and the numerical aperture of the
light-receiving section on the side of the return path are
represented by .lamda. and NA' respectively as described above, a
beam spot on the light-receiving element has a radius R that is
expressed by: R=k.lamda./NA' Expression 1 where k represents a
constant determined by the position that is defined as the beam
waist. According to the reference document, when it is assumed that
a radius where intensity is e.sup.-2 times the intensity at the
center is defined as the beam waist, the radius of the beam spot is
0.41.lamda./NA', and the constant k in Expression 1 is therefore
0.41. The document indicates that the coefficient is always 0.41
regardless of the wavelength .lamda. of the light source and the
numerical aperture NA' of the lens.
However, as shown in FIG. 3, the optical energy P(r) at a radius r
(which substantially equals 1.66 .mu.m) where intensity is e.sup.-2
(which substantially equals 0.135) times the intensity at the
center does not reach 90% of the energy of the spot of reflected
light. Therefore, in order to receive 90% or more of the reflected
light with a sufficient margin, in the example shown in FIG. 3, the
light-receiving region must have a radial position where optical
intensity is 0.05 times the intensity at the center or a radius r
that is substantially equal to or greater than 2 .mu.m. In order to
obtain the value of the coefficient k that satisfies such a
condition, 2 .mu.m, 0.405 .mu.m, and 0.1 are substituted for R,
.lamda., and NA' in Expression 1 respectively, .lamda.=0.405 and
NA'=0.1 being prerequisites for FIG. 3. Then, k=0.5. That is, in
order to receive the beam of the reflected light spot effectively,
the radius R of the circular light-receiving region is desirably
set at 0.5 (.lamda./NA') or more. As described above, this
coefficient always holds regardless of the values of the laser
wavelength .lamda. and the numerical aperture NA'.
FIG. 4 shows the intensity distribution and optical energy
distribution of defocused light that is received on the
light-receiving surfaces of the first and second light-receiving
sections 27 and 29 through the return path optical system of the
optical head 1. In FIG. 4, distances (.mu.m) from the center of the
light-receiving region of the first light-receiving section 27 in
the radial (r) direction thereof are plotted along the abscissa
axis, and the intensity I(r) of the focused light in each position
in the radial (r) direction and an integrated value of optical
intensities (optical energy P(r)) from the center of the region at
each of the radial positions are plotted along the ordinate axis in
a normalized manner.
The example shown in FIG. 4 is on assumptions that the distance d
between layers among the multiplicity of recording layers of the
multi-layer optical disk 15 is 10 .mu.m and that reflected light
which is defocused from the focus of a reproduced layer by 20 .mu.m
is received in the same position under the same condition as in
FIG. 3. It is assumed that a lateral magnification .beta. of the
side of the return path is 8.5; the numerical aperture NA' of the
side of the return path is 0.1; and the refractive index of a
light-transmitting layer constituting a section between recording
layers is 1.58.
Let us assume now that d represents the distance between a
reproduced layer and a layer adjacent thereto among the plurality
of recording layers; n represents the refractive index of the
light-transmitting layer between the recording layers; .beta.
represents the lateral magnification of the side of the return
path; and NA' represents the numerical aperture of the
light-receiving section on the side of the return path. Then,
reflected light from the adjacent layer impinges upon the
light-receiving element on the return path side in a state of
defocus, and the amount of the defocus is 2d.beta..sup.2NA'/n. This
is attributable to the fact that the displacement of a defocused
image point in the direction of the optical axis is equivalent to
the amount of defocus at the side of the object point multiplied by
a longitudinal magnification .beta..sup.2 (reference document:
"Introduction to Optics", Toshiro Kishikawa, The Optronics Co.,
Ltd. Pp. 26-31, 1990).
Since the radius of the beam incident upon the light-receiving
element is proportionate to the amount of defocus, when the
proportionality factor is represented by k', the radius R of the
beam spot on the light-receiving element is expressed by:
R=k'd.beta..sup.2NA'/n Expression 2
In this case, the object of the discussion is the optical intensity
distribution in an area beyond the focus position of the lens, and
the wavelength of the light source does not contribute so much to
the optical intensity distribution different from the case of
Expression 1. As apparent from Expression 2, the radius R of the
beam spot is the product of the amount of defocus at the side of
the image point (the amount of defocus at the side of the medium
d/n multiplied by a longitudinal magnification .beta..sup.2) and
the numerical aperture NA' on the side of the image point, and the
value can be calculated on the basis of geometrical optics.
In order to keep noise light entering the first light-receiving
section 27 at 5% or less of the total optical energy P of the
defocused light (a range in which no abrupt increase occurs in the
quantity of the entering noise light), the first light-receiving
section 27 must be a region in which the optical intensity I(r) is
equal to or greater than 0.96 times the intensity at the center
thereof when the section is circular as shown in FIG. 4. That is,
the section must be about 11 .mu.m or less in terms of the radius
R. When the radius R=11 .mu.m and the parameters defined as
prerequisites of FIG. 4 are substituted in Expression 2, the
expression is changed into k'=0.24. That is, the radius of the
first light-receiving section 27 is desirably set at
(0.24d.beta..sup.2NA'/n) or less to prevent a crosstalk between the
adjacent layers effectively. A geometrical-optical study revealed
that the coefficient k' always holds as a constant independently of
the values of the parameters associated with the amount of
defocus.
The numerical aperture NA' of the light-receiving section on the
side of the return path can be rewritten as NA'=NA/.beta. using the
numerical aperture NA of the objective lens 13 and the lateral
magnification .beta. of the optical system. Results of those
discussions indicate that an optimum range of the surface area of
the first light-receiving section 27 which has a light-receiving
region in the innermost circumferential part is expressed by:
.pi.(0.5.lamda./(NA/.beta.)).sup.2.ltoreq.S.sub.1.ltoreq..pi.(0.24d.beta.-
NA/n).sup.2 Expression 3 where S.sub.1 represents the surface area
of the section 27.
In order to provide a function of eliminating unwanted inter-layer
crosstalks from focused light, it is desirable that the
light-receiving region of the first light-receiving section 27 has
a circular shape.
Referring now to the purpose of the second light-receiving section
29 which is concentric with the circular light-receiving region of
the first light-receiving section 27 and disposed adjacent to the
outer circumference of the first light-receiving section 27, the
section positively receives reflected light from recording layers
other than a reproduced layer and performs a differential operation
between such components and a signal from the reproduced layer to
achieve an improvement in an S/N ratio that is the ratio between RF
signal components and noise signal components. Therefore, the
light-receiving region of the second light-receiving section 29
must have a size to allow efficient reception of reflected light
from recording layers other than a reproduced layer.
However, noise signal components to be eliminated as inter-layer
crosstalks are not limited to inter-layer crosstalks from a
recording layer adjacent to a reproduced layer, and it is necessary
to eliminate noise signal components originating in inter-layer
crosstalk signals between reflected light from another recording
layer that is adjacent to the above-mentioned recording layer and
still another recording layer that is adjacent to the other
recording layer. Let us now call the reproduced layer "first
recording layer", the recording layer adjacent to the reproduced
layer "second recording layer, and so on. Then, reflected light
from an m-th recording layer impinges upon the light-receiving
element 25 out of focus (in a state of defocus), and the amount of
defocus is expressed by 2(m-1)d.beta..sup.2NA'/n using the same
parameters as above. The radius of a light beam incident upon the
light-receiving element 25 is proportionate to the amount of
defocus of the same. Therefore, when a constant k'' is put to
represent the proportionality factor including the coefficient "2"
in the above expression, the radius R of a beam spot on the
light-receiving element 25 can be expressed by:
R=k''(m-1)d.beta..sup.2NA'/n Expression 4
As apparent from FIG. 4, in order to detect reflected light from
the m-th recording layer with an optical energy intensity I(r) of
0.5 (=50%), an m-th light-receiving region of the second
light-receiving section 29 must have a radius of about 50 .mu.m or
more on an assumption that the region is circular. When the radius
R=50 .mu.m and the parameters defined as prerequisites of FIG. 4
are substituted in Expression 4, the expression is changed into
k=1.1. That is, the radius R of the m-th (m.gtoreq.2)
light-receiving region of the second light-receiving section 29 is
desirably set at (1.1(m-1)d.beta..sup.2NA'/n) or more to detect
noise signals included in reflected light from the m-th recording
layer effectively, the light-receiving region of the first
light-receiving section 27 being the first light-receiving region
(m=1).
The numerical aperture NA' of a light-receiving section on the side
of the return path can be expressed by NA'=NA/.beta.. Then, when
S.sub.m represents the sum of surfaces areas of light-receiving
regions from the first light-receiving section 27 up to the m-th
light-receiving region, an optimum range of S.sub.m can be
expressed by: S.sub.m.gtoreq..pi.(1.1(m-1)d.beta.NA/n).sup.2
Expression 5
It is desirable that the m-th light-receiving region in the
outermost circumferential section has a circular shape for the same
reason as for the first light-receiving section 27 in the innermost
circumferential section. However, the restriction on the shape of
this light-receiving region is not as strict as that on the shape
of the first light-receiving section 27 because any light-receiving
part other than the first light-receiving section 27 is used for
detecting noise signal components. The light-receiving region may
therefore have a different shape such as a square. The function of
the light-receiving element 25 is not degraded even in such a case.
Therefore, the shape of the second light-receiving section 29 may
be appropriately decided in accordance with restrictions on the
design of the light-receiving element 25 and the optical head
1.
As described with reference to FIGS. 3 and 4, the radius R of the
light-receiving region of the first light-receiving section 27
satisfying Expression 1 is in the range from 2 to 11 .mu.m where
.lamda.=0.405 .mu.m; NA=0.85; .beta.=8.5; d=10 .mu.m; and n=1.58.
Therefore, the surface area S.sub.1 of the light-receiving region
of the first light-receiving section 27 ranges from 12.6 to 380
(.mu.m.sup.2). The radius R of the light-receiving region of the
second light-receiving section 29 is 50.3 .mu.m when m=2, and the
sum S.sub.m of the surface areas of the light-receiving regions of
the first and second light-receiving sections 27 and 29 is 7948
(.mu.m.sup.2) or more.
Next, a description will now be made with reference to FIG. 5 on a
method of extracting an RF signal including information recorded on
a reproduced layer. FIG. 5 shows a differential amplification
circuit 31 which is provided in the light-receiving element 25 and
which extracts an RF signal including information recorded in the
multi-layer optical disk 15 from electrical signals output by the
first and second light-receiving sections 27 and 29. The
differential amplification circuit 31 has an operational amplifier
37 and resistors 33, 34, 35 and 36 used for protecting the
operational amplifier 37 from inputs and for determining the
amplification factor of the same. One terminal of the resistor 34
is connected to an output terminal, which is not shown, of the
first light-receiving section 27, and another terminal of the
resistor 34 is connected to a non-inverting input terminal (+) of
the operational amplifier 37. One terminal of the resistor 33 is
connected to an output terminal, which is not shown, of the second
light-receiving section 29, and another terminal of the resistor 33
is connected to an inverting input terminal (-) of the operational
amplifier 37. One terminal of the resistor 35 is connected to an
output terminal 38 of the operational amplifier 37, and another
terminal of the resistor 35 is connected to the inverting input
terminal (-) of the operational amplifier 37. One terminal of the
resistor 36 is connected to the non-inverting input terminal (+) of
the operational amplifier 37, and another terminal of the resistor
36 is connected to the ground (reference potential). The resistors
33, 34, 35 and 36 have the same resistance. Obviously, the
resistance of each of the resistors may be set at a predetermined
value to set the amplification factor of the operational amplifier
37 at a predetermined value.
A method of optical recording and reproduction utilizing the
differential amplification circuit 31 will now be described with
reference to FIG. 5. The light-receiving region of the first
light-receiving section 27 of the light-receiving element 25 has a
shape and a surface area which are determined such that reflected
light from a reproduced layer of the multi-layer optical disk 15
can be received when the disk is irradiated with laser light.
However, as shown in FIG. 4, some of the light received by the
light-receiving region (having a radius R=2 to 11 .mu.m) of the
first light-receiving section 27 includes noise components which
are generated in reflected light (return light) from recording
layers other than the reproduced layer. Referring to FIG. 4, for
example, the light constituting noise components included in the
light received by the first light-receiving section 27 is
equivalent to about 0.5% of optical energy P(r) received by the
second light-receiving section 29. Therefore, an electrical signal
obtained by photoelectrically converting the light received by the
first light-receiving section 27 includes an RF signal and a noise
signal originating in inter-layer crosstalks that occur between the
reflected light from the reproduced layer and the return light from
recording layers other than the reproduced layer. As a result, the
frequency of the electrical signal output from the first
light-receiving section 27 includes the frequency of the noise
signal (a low frequency) and the frequency of the RF signal (a high
frequency).
The shape and the surface area of the light-receiving region of the
second light-receiving section 29 are determined such that
reflected light from recording layers other than the reproduced
layer can be received. Thus, an electrical signal obtained by
photoelectrically converting the light received by the second
light-receiving section 29 includes only a noise signal which
causes inter-layer crosstalks between the reflected light from the
reproduced layer and the return light from recording layers other
than the reproduced layer. As a result, the frequency of the signal
output from the second light-receiving section 29 is equivalent to
the frequency of the noise signal. Therefore, the frequency of the
electrical signal output from the first light-receiving section 27
includes the frequency of the noise signal output from the second
light-receiving section 29. The electrical signal including an RF
signal and a noise signal obtained by photoelectrical conversion at
the first light-receiving section 27 is input to the non-inverting
input terminal (+) of the operational amplifier 37 through the
resistor 34. The noise signal obtained by photoelectrical
conversion at the second light-receiving section 29 is input to the
inverting input terminal (-) of the operational amplifier 37
through the resistor 33. The operational amplifier 37 performs a
differential operation between the electrical signal and the noise
signal to extract only the RF signal and output the RF signal from
the output terminal 38.
As thus described, the optical head 1 of the present embodiment has
the light-receiving element 25 including the circular first
light-receiving section 27 and the second light-receiving section
29 disposed adjacent to the outer circumference of the first
light-receiving section 27. The light-receiving element 25 is
disposed in the vicinity of the focusing point of the return path
optical system such that the center of the first light-receiving
section 27 coincides with the focal point section. As a result, the
first light-receiving section 27 can receive reflected light
including an RF signal reflected by a reproduced layer of the
multi-layer optical disk 15 and a noise signal attributable to an
inter-layer crosstalk, and the second light-receiving section 29
can receive reflected light including a noise signal reflected by
information recording layers other than the reproduced layer.
Therefore, an RF signal can be reproduced with high quality by
performing a differential operation by the differential
amplification circuit 31 between an electrical signal obtained by
photoelectrically converting the light received by the first
light-receiving section 27 and a noise signal obtained by
photoelctrically converting the light received by the second
light-receiving section 29. Since the light-receiving regions of
the first and second light-receiving sections 27 and 29 can be
formed with optimum surface areas taking the optical system of the
optical head 1 and the inter-layer distance of the multi-layer
optical disk 15 into consideration, the optical head 1 and the
light-receiving elements 25 can be made compact.
FIG. 6 shows a schematic configuration of an optical
recording/reproducing apparatus 50 loaded with an optical head 1
according to the present embodiment. As shown in FIG. 6, the
optical recording/reproducing apparatus 50 has a spindle motor 52
for rotating a multi-layer optical disk 15, the optical head 1 for
irradiating the multi-layer optical disk 15 with a laser beam and
for receiving reflected light from the same, a controller 54 for
controlling the operation of the spindle motor 52 and the optical
head 1, a laser driving circuit 55 for supplying a laser driving
signal to the optical head 1, and a lens driving circuit 56 for
supplying a lens driving signal to the optical head 1.
The controller 54 includes a focus servo following circuit 57, a
tracking servo following circuit 58, and a laser control circuit
59. When the focus servo following circuit 57 is activated, an
information recording surface of the multi-layer optical disk 15
that is rotating is focused. When the tracking servo following
circuit 58 is activated, a laser beam spot automatically follows up
an eccentric signal track of the multi-layer optical disk 15. The
focus servo following circuit 57 and the tracking servo following
circuit 58 are provided with an automatic gain control function for
automatically adjusting a focus gain and a tracking gain,
respectively. The laser control circuit 59 is a circuit for
generating the laser driving signal supplied from the laser driving
circuit 55, and the circuit generates a proper laser driving signal
based on recording condition setting information that is recorded
in the multi-layer optical disk 15.
It is not essential that the focus servo following circuit 57, the
tracking servo following circuit 58, and the laser control circuit
59 are circuits incorporated in the controller 54, and the circuits
may be components separate from the controller 54. Further, it is
not essential that the circuits are physical circuits, and they may
be programs executed in the controller 54.
A modification of the embodiment will now be described with
reference to FIGS. 7 and 8. FIG. 7 shows a configuration of a
light-receiving section of a light-receiving element 25 of the
present modification. As shown in FIG. 7, the light-receiving
element 25 has a first light-receiving section 27 and a second
light-receiving section 29 whose light-receiving region has three
divisions. The second light-receiving section 29 has
light-receiving regions 29a, 29b and 29c which are concentric with
the first light-receiving section 27. The light-receiving region
29a is formed around the outer circumference of a light-receiving
region of the first light-receiving section 27 in the form of a
circle concentric therewith. The light-receiving region 29b is
formed around the outer circumference of the light-receiving region
29a in the form of a concentric circle. The light-receiving region
29c, which has a square outer circumference, is formed around the
outer circumference of the light-receiving region 29b. An
insulation region 26 is provided between the light-receiving
regions of the first light-receiving section 27 and the second
light-receiving section 29 to keep them insulated from each other.
A wiring region 28, which extends from the center of the element in
the radial direction thereof, is formed in the light-receiving
regions 29a, 29b and 29c. The wiring region 28 is provided to form
wirings for connecting the first and second light-receiving
sections 27 and 29 with a differential amplification circuit
31.
The surface areas of the light receiving regions of the first and
second light-receiving sections 27 and 29 are adjusted according to
Expressions 1 and 5 described in the embodiment. The parameter m in
Expression 5 has values 2, 3 and 4 for the light-receiving regions
29a, 29b and 29c, respectively. Therefore, when the values of the
other parameters in the above embodiment are used, the radius R of
the light-receiving region 29a is 50.3 .mu.m or more according to
Expression 5. Similarly, the radius R of the light-receiving region
29b is 100.6 .mu.m or more, and the length of one side of the
light-receiving region 29c is 301.8 (=150.9.times.2) .mu.m.
Therefore, the light-receiving element 25 has a total area S.sub.m
of 71536 (.mu.m.sup.2) or more.
Next, a description will now be made with reference to FIG. 8 on a
method of extracting an RF signal including information recorded on
a reproduced layer. FIG. 8 shows a circuit configuration of the
differential amplification circuit 31 in the present modification.
As shown in FIG. 8, the differential amplification circuit 31 has a
noise signal selection circuit 39, an operational amplifier 37 and
resistors 33a, 33b, 33c, 34, 35 and 36 used for protecting inputs
to the operational amplifier 37 and for determining the
amplification factor of the same. Noise signals based on light
received by the light-receiving regions 29a, 29b and 29c can be
selectively input to the operational amplifier 37 of the
operational amplification circuit 31 by a noise signal selection
circuit 39.
The noise signal selection circuit 39 has switches 41a, 41b and
41c. One terminal of each of the switches 41a, 41b and 41c is
connected to an output terminal (which is not shown) of the
light-receiving regions 29a, 29b and 29c, respectively. Another
terminal of each of the switches 41a, 41b and 41c is connected to
one terminal of the resistors 33a, 33b and 33c, respectively.
Another terminal of each of the resistors 33a, 33b and 33c is
connected to an inverting input terminal (-) of the operational
amplifier 37. Another terminal of the resistor 34 is connected to a
non-inverting input terminal (+) of the operational amplifier 37.
One terminal of the resistor 35 is connected to an output terminal
38 of the operational amplifier 37, and another terminal of the
resistor 35 is connected to the inverting input terminal (-) of the
operational amplifier 37. One terminal of the resistor 36 is
connected to the non-inverting input terminal (+) of the
operational amplifier 37, and another terminal of the resistor 36
is connected to the ground. The resistors 33a, 33b, 33c, 34, 35 and
36 have the same resistance. Obviously, the resistance of each of
the resistors may be set at a predetermined value respectively to
set the amplification factor of the operational amplifier 37 at a
predetermined value.
Noise signals to be eliminated when an RF signal is being
reproduced are not limited to noise signals attributable to
inter-layer crosstalks that occur between reflected light from a
reproduced layer and reflected light from a recording layer
adjacent to the reproduced layer. In the present modification, the
second light-receiving section 29 is provided with the three
light-receiving regions 29a, 29b and 29c such that reflected light
from each of recording layers of a multi-layer optical disk 15 can
be received. Further, the switches 41a, 41b and 41c are connected
to the light-receiving regions 29a, 29b and 29c, respectively, to
allow noise signals output by the light-receiving regions 29a, 29b
and 29c to be selectively input to the operational amplifier 37. As
a result, noise signals included in light received by the first
light-receiving section 27 can be sufficiently eliminated to
reproduce an RF signal of high quality. It is not essential that
noise signals are input to the operational amplifier 37 from any
one of the light-receiving regions 29a, 29b and 29c, and two or all
of the switches 41a, 41b and 41c may be turned on.
As thus described, according to the present modification, since
noise signals that degrade the S/N ratio of a reproduced signal can
be selectively eliminated, an RF signal of higher quality can be
reproduced.
Another modification of the embodiment will now be described. With
the differential amplification circuit 31 of the above
modification, an RF signal can be reproduced by performing a
differential operation between an electrical signal output by the
first light-receiving section 27 and a noise signal selected by the
noise signal selection circuit 39 from among outputs of the
light-receiving regions 29a, 29b, and 29c. On the contrary, a
differential amplification circuit 31 of the present modification
is characterized in that it has a switch (not shown) which
switchably connects the other terminal of the resistor 34 to either
of the non-inverting input terminal (+) or the inverting input
terminal (-) of the operational amplifier 37 with one terminal of
the resistor 36 kept connected to the non-inverting input terminal
(+) of the operational amplifier 37 and an input terminal (not
shown) to which an output signal from a control circuit or logic
circuit (not shown) is input. The opening and closing of the switch
is controlled by logical inputs from the control circuit.
When the other terminal of the resistor 34 is connected to the
non-inverting input terminal (+) of the operational amplifier 37,
the differential amplification circuit 31 operates as a
differential amplification circuit just as in the above-described
modification. The circuit 31 operates as an adding circuit
(arithmetic circuit section) when the other terminal of the
resistor 34 is connected to the inverting input terminal (-) of the
operational amplifier 37. When the differential amplification
circuit 31 is operated as an adding circuit, an electrical signal
output by the first light-receiving section 27 and a noise signal
selected by the noise signal selection circuit 39 from among
outputs of the light-receiving regions 29a, 29b and 29c can be
added as occasions demand, and a signal (voltage) obtained by the
addition can be output from the output terminal 38. Therefore, the
light-receiving element 25 having the first light-receiving section
27 and the second light-receiving section 29 with the divisional
light-receiving regions 29a, 29b and 29c can be used as a
light-receiving element for monitoring laser power when connected
with the differential amplification circuit 31 of the present
modification. As a result, the manufacturing cost of the optical
head 1 can be reduced because a single light-receiving element can
be used to provide functions of both a light-receiving element 25
for extracting an RF signal and a power-monitoring photodiode 11
for front-end monitoring.
* * * * *